henning - Does the writing system have an impact on the language?

Yesterday evening my (Chinese) collegue and I discussed what would happen if China in fact abondoned 汉字 and switched to Pinyin only.

We agreed that Pinyin is virtually unreadable - pure pain, due to all the ambiguities and homophones. But would this in fact foster a slow change of the language? At least the written language can be expected to be altered - more multi syllable words, more redundancy, more words with distinct pronounciation.

And would this also feed through into the spoken language? Or vice versa: Does Chinese have so many homophones because of its semantically rich characters? Changye's posts on the development of tones and sounds seem to support this hypothesis (e.g. sounds that fell together over time, tones that vanished - all more widespread in Mandarin than in dialects that are primarily passed down in spoken form).

My collegue made a good point - if this would be true, you should be able to observe similar effects in Japanese, as Japan has already left pure 汉字 behind by introducing hiragana and katagana. Did this have an impact on the language?

Any linguist out there who knows more?